void main(List<String> args) {
  //identical 判断两个对象是否指向同一个内存地址
  var o1 = new Object();
  var o2 = new Object();
  print(identical(o1, o2));

  var o3 = const Object();
  var o4 = const Object();
  print(identical(o3, o4));

  print(identical([2], [2]));

  print(identical(const [2], const [2]));
}
